Created attachment 504872 [details] Screenshot STR: 1. Open a new window. (see the screenshot). The button would move back to the right side if you open enough tabs to cause an overflow.
I can't reproduce this. Can you verify that this happens in a clean profile? We even have a hack in our tab strip code to circumvent this XBL bug. XBL bug: bug 472020 hack to prevent new tab button from appearing on the left: bug 457651
Can't repro even on the same profile any more!